The invention relates generally to pulse width modulated power supplies, and deals more particularly with circuitry to prevent overshoot during start up or a single or plural phase power supply and when one phase or a plural phase power supply resumes operation after dropping out.
A standard pulse width modulated power supply for a single phase power source comprises a power train transformer having primary and secondary windings. A full wave rectifier bridge is connected to the power source. One end of the primary winding is connected to an output of the full wave rectifying bridge and the other end is connected to a semiconductor switch. The switch is repeatedly turned on and off to develop a high frequency voltage across the primary winding. A half or full wave rectifier bridge is connected across the secondary winding and charges an output capacitor to supply output DC voltage.
One prior art technique to control the switch uses a differential amplifier that receives at one input a fixed reference voltage and at the other input a fraction of the DC output voltage. A feedback capacitor is connected between the output of the amplifier and one of the inputs to filter out 120 hertz ripple. The output of the differential amplifier is called an "error voltage" and is proportional (after the time constant of the feedback capacitor is satisfied) to the difference between the fractional output voltage and the reference voltage. The voltage at the output of the amplifier (developed on the feedback capacitor) is supplied to one input of a comparator. The other input is supplied by the voltage across a timing capacitor which is charged by rectified current from a secondary of another current sense transformer. When the voltage across the timing capacitor just exceeds the error voltage, the comparator resets a flip flop which shuts off the switch (until the next cycle of a clock which sets the flip flop). Thus, the greater the error voltage the greater the duty cycle. Also, the flip flop activates a transistor to reset the timing capacitor in preparation for the next cycle.
The time constant of the differential amplifier and associated feedback capacitor is large, for example, 100 milliseconds, because it must be much larger than the period of the 120 hertz ripple for proper operation of a bulkless AC/DC converter. This leads to an overshoot problem during start up of a single or plural phase power supply and during resumption of one input phase in a plural phase power source.
Accordingly, a general object of the present invention is to provide a single or plural phase, pulse width modulated power supply which avoids overshoot during start up and when one phase or a plural phase power source resumes after dropping out.